A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by a process known as “the photovoltaic effect.” When exposed to light, the solar cell can generate and support an electric current without being attached to any external voltage source.
The most common solar cell consists of a p-n junction 110 fabricated from semiconductor materials (e.g., silicon), such as in a solar cell 100 shown in FIG. 1. For example, the p-n junction 110 includes a thin wafer consisting of an ultra-thin layer of n-type silicon on top of a thicker layer of p-type silicon. Where these two layers are in contact, an electrical field (not shown) is created near the top surface of the solar cell 100, and a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the p-n junction 110) into the region of low electron concentration (the p-type side of the p-n junction 110).
The p-n junction 110 is encapsulated between two conductive electrodes 101a, 101b. The top electrode 101a is either transparent to incident (solar) radiation or does not entirely cover the top of the solar cell 100. The electrodes 101a, 101b can serve as ohmic metal-semiconductor contacts that are connected to an external load 30 that is coupled in series. Although shown as resistive only, the load 30 can also include both resistive and reactive components.
Typically, multiple solar cells 100 can be coupled (in series and/or parallel) together to form a solar panel 10 (shown in FIG. 2). With reference to FIG. 2, a typical installation configuration using at least one solar panel 10 is shown. The solar panels 10 can be connected either in parallel as shown in FIG. 2, series, or a combination thereof, and attached to a load, such as an inverter 31. The inverter 31 can include both resistive and reactive components.
Returning to FIG. 1, when a photon hits the solar cell 100, the photon either: passes straight through the solar cell material—which generally happens for lower energy photons; reflects off the surface of the solar cell; or preferably is absorbed by the solar cell material—if the photon energy is higher than the silicon band gap—generating an electron-hole pair.
If the photon is absorbed, its energy is given to an electron in the solar cell material. Usually this electron is in the valence band and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to the electron by the photon “excites” the electron into the conduction band, where it is free to move around within the solar cell 100. The covalent bond that the electron was previously a part of now has one fewer electron—this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the hole, leaving another hole behind. In this way, a hole also can move effectively through the solar cell 100. Thus, photons absorbed in the solar cell 100 create mobile electron-hole pairs.
The mobile electron—hole pair diffuses or drifts toward the electrodes 101a, 101b. Typically, the electron diffuses/drifts towards the negative electrode, and the hole diffuses/drifts towards the positive electrode. Diffusion of carriers (e.g., electrons) is due to random thermal motion until the carrier is captured by electrical fields. Drifting of carriers is driven by electric fields established across an active field of the solar cell 100. In thin film solar cells, the dominant mode of charge carrier separation is drifting, driven by the electrostatic field of the p-n junction 110 extending throughout the thickness of the thin film solar cell. However, for thicker solar cells having virtually no electric field in the active region, the dominant mode of charge carrier separation is diffusion. The diffusion length of minor carriers (i.e., the length that photo-generated carriers can travel before they recombine) must be large in thicker solar cells.
Ultimately, electrons that are created on the n-type side of the p-n junction 110, “collected” by the p-n junction 110, and swept onto the n-type side can provide power to the external load 30 (via the electrode 101a) and return to the p-type side (via the electrode 101b) of the solar cell 100. Once returning to the p-type side, the electron can recombine with a hole that was either created as an electron-hole pair on the p-type side or swept across the p-n junction 110 from the n-type side.
As shown in FIG. 1, the electron-hole pair travels a circuitous route from the point the electron-hole pair is created to the point where the electron-hole pair is collected at the electrodes 101a, 101b. Since the path traveled by the electron-hole pair is long, ample opportunity exists for the electron or hole to recombine with another hole or electron, which recombination results in a loss of current to any external load 30. Stated in another way, when an electron-hole pair is created, one of the carriers may reach the p-n junction 110 (a collected carrier) and contribute to the current produced by the solar cell 100. Alternatively, the carrier can recombine with no net contribution to cell current. Charge recombination causes a drop in quantum efficiency (i.e., the percentage of photons that are converted to electric current when the solar cell 100), and, therefore, the overall efficiency of the solar cell 100.
The cost of the solar cell 100 or the solar panel 10 is typically given in units of dollars per watts of peak electrical power that can be generated under normalized conditions. High-efficiency solar cells decrease the cost of solar energy. Many of the costs of a solar power system or plant are proportional to the number of solar panels required as well as the (land) area required to mount the panels. A higher efficiency solar cell will allow for a reduction in the number of solar panels required for a given energy output and the required area to deploy the system. This reduction in the number of panels and space used might reduce the total plant cost, even if the cells themselves are more costly.
The ultimate goal is to make the cost of solar power generation comparable to, or less than, conventional electrical power plants that utilize natural gas, coal, and/or fuel oil to generate electricity. Unlike most conventional means of generating electric power that require large centralized power plants, solar power systems can be deployed at large centralized locations by electric utilities, on commercial buildings to help offset the cost of electric power, and even on a residence by residence basis.
Recent attempts to reduce the cost and increase the efficiency of solar cells include testing various materials and different fabrication techniques used for the solar cells. Another approach attempts to enhance the depletion region formed around the p-n junction 110 for enhancing the movement of charge carriers through the solar cell 100. For example, see U.S. Pat. No. 5,215,599, to Hingorani, et al. (“Hingorani”), filed on May 3, 1991, and U.S. Pat. No. 8,466,582, to Fornage (“Fornage”), filed on Dec. 2, 2011, claiming priority to a Dec. 3, 2010 filing date, the disclosures of which are hereby incorporated by reference in their entireties and for all purposes.
However, these conventional approaches for enhancing the movement of charge carriers through the solar cell 100 require a modification of the fundamental structure of the solar cell 100. Hingorani and Fornage, for example, disclose applying an external electric field to the solar cell using a modified solar cell structure. The application of the external electric field requires a voltage to be applied between electrodes inducing the electric field (described in further detail with reference to equation 2, below). Without modifying the fundamental structure of the solar cell 100, applying the voltage to the existing electrodes 101a, 101b of the solar cell 100 shorts the applied voltage through the external load 30. Stated in another way, applying voltage to the electrodes 101a, 101b of the solar 100 is ineffective for creating an external electric field and enhancing the movement of charge carriers. Accordingly, conventional approaches—such as disclosed in Hingoriani and Fornage—necessarily modify the fundamental structure of the solar cell 100, such as by inserting an external (and electrically isolated) set of electrodes on the base of the solar cell 100. There are several disadvantages with this approach.
For example, the external electrodes must be placed on the solar cell 100 during the fabrication process—it is virtually impossible to retrofit the external electrodes to an existing solar cell or panel. This modification to the fabrication process significantly increases the cost of manufacturing and decreases the manufacturing yield. Additionally, placement of the external electrodes over the front, or incident side, of the solar cell 100 reduces the optical energy which reaches the solar cell 100, thereby yielding a lower power output.
As a further disadvantage, to yield significant improvements in power output of the solar cell 100, sizeable voltages must be applied to the external electrodes of the solar cell 100. For example, Fornage discloses that voltages on the order of “1,000's” of volts must be placed on the external electrodes for the applied electric field to be effective and increase the power output of the solar cell 100. The magnitude of this voltage requires special training for servicing as well as additional high voltage equipment and wiring that does not presently exist in existing or new solar panel deployments. As an example, an insulation layer between the external electrodes and the solar cell 100 must be sufficient to withstand the high applied voltage. In the event of a failure of the insulation layer, there is a significant risk of damage to not only the solar cell 100, but also all solar panels 10 connected in series or parallel to the failed solar cell as well as the external load 30 (or the inverter 31).
As a further disadvantage, varying illumination conditions (e.g., due to cloud coverage of the sun and/or normal weather fluctuations) can cause instability in the power output of conventional solar cells and solar panels. For example, with reference to FIG. 2, the inverter 31 typically requires a static, non-varying voltage and current input. As shown in FIG. 2, the solar panels 10 provide the input voltage and current to the inverter 31. However, time-varying illumination conditions can cause the output from solar panels 10 to fluctuate (e.g., on the order of seconds or less). The fluctuation of the voltage and current supplied to the inverter 31 compromises the quality of the power output by the inverter 31, for example, in terms of frequency, voltage, and harmonic content. Conventional efforts to combat varying illumination conditions include placing batteries or capacitors at the input of the inverter 31 and, unfortunately, only minimize these variations.
In view of the foregoing, a need exists for an improved solar cell system and method for increased efficiency and power output, such as with increased mobility of electron-hole pairs, in an effort to overcome the aforementioned obstacles and deficiencies of conventional solar cell systems.
It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It also should be noted that the figures are only intended to facilitate the description of the preferred embodiments. The figures do not illustrate every aspect of the described embodiments and do not limit the scope of the present disclosure.